For the structural adhesive bonding of components, i.e. for bonding with very high (>10 MPa) peel adhesions, as in automotive engineering, for example, it is usual to use epoxide-based liquid adhesives. Under the complex control of metering machines, these adhesives are applied, and possess no initial bond strength, meaning that the components must be held in position over the curing period. There are a few systems which are obtained via inherently pressure-sensitively adhesive acrylates, but they lose their pressure-sensitive adhesiveness when the epoxide fraction goes beyond 50 wt %. Particularly high shear strengths, however, are obtained with very high epoxide fractions. A higher fraction of epoxide and hence a better peel adhesion are therefore always at the expense of the pressure-sensitive adhesiveness. Desirable adhesives are those which unite the positive qualities of a high epoxide fraction and high pressure-sensitive adhesiveness. For particularly simple application, in particular, adhesive tapes are preferred.
In the prior art there are a variety of approaches discernible to realizing a high epoxide fraction in conjunction with pressure-sensitive adhesiveness:
DE 10 2004 031 188 A1 discloses adhesives which consist at least of a) an acid-modified or acid anhydride-modified vinylaromatic block copolymer and b) an epoxide-containing compound. The chemical crosslinking of the resins with the elastomers produces very high strengths within the adhesive film. To increase the adhesion, the addition of tackifier resins compatible with the elastomer block of the block copolymers is also possible. The acid-modified and/or acid anhydride-modified elastomers and epoxy resins are used in a proportion such that the molar fraction of epoxide groups and anhydride groups is just equivalent. In this context, when the common elastomers with only low levels of modification are used, and when relatively low molecular mass epoxy resins having a low epoxide equivalent are employed, the amounts of epoxy resin used are only very small—below 10 wt %, based on the modified styrene block copolymer. In this way, essentially, the epoxy resin acts as a crosslinking agent for the modified elastomers.
U.S. Pat. Nos. 6,294,270 B, 6,423,367 B and 6,489,042 B describe crosslinkable mixtures of epoxidized vinylaromatic block copolymers and epoxy resins for purposes including their use as adhesives for the bonding of two electronic components. Described primarily is crosslinking through irradiation with UV light. For this purpose, here, a very high fraction of more than 90 wt % of the epoxidized vinylaromatic block copolymer has proved to be the best solution. Here again, essentially, the epoxy reisn is acting as a crosslinking agent for the modified elastomers. From these four abovementioned specifications it is apparent that there must in each case be a homogeneous blend of reactive resin and modified block copolymer, distinguished by high compatibility between these at least two components, so that even after crosslinking, the epoxy resin is present in molecularly dispersed form in the elastomer phase. Since the epoxy resin acts as crosslinking agent, there can be no formation of crosslinked epoxide phases. In their function as crosslinking agents, epoxides are used at not more than 10% in pressure-sensitive adhesive systems.
DE 10 2011 105 209 A1 describes a pressure-sensitively adhesive tape for structural adhesive bonds. In the prior art described therein it is expressly noted that there is no known pressure-sensitively adhesive system which on exposure to high temperature allows a structural bond free from creases and bubbles. Above all it is noted that bubble-free and channel-free products were not produced from solvent-based pressure sensitive adhesives. The problem was solved by way of a reactive thick-film polymer which as well as radically polymerizable acrylates contains 5-15% of solid bisphenol A epoxide and is crosslinked using UV light to such an extent as to obtain a slightly pressure-sensitively adhesive tape.
From this it is apparent that for pressure-sensitively adhesive systems, inherently pressure-sensitively adhesive acrylate-based elastomers are needed, and that these elastomers can be crosslinked with up to 15% of epoxide.
EP 1 028 151 B1 describes an acrylate-based pressure sensitive epoxide adhesive which on the one hand is made pressure-sensitively adhesive by maleic anhydride copolyacrylates, while on the other hand the epoxide fraction can be increased, surprisingly, to up to 50%. It is maintained there that after curing, the shear strength goes up with the amount of epoxide, but the pressure-sensitive adhesiveness is lost. Through the use of maleic anhydride copolyacrylates as tackifier resin, however, it is possible to achieve epoxide fractions of up to 50 wt %, although preference is given to using 20 to 40 wt %, and the examples use not more than 25 wt %.
EP 0 386 909 A1 describes a copolyacrylate-based prepolymer which is preparable by UV radiation, which is mixed with a latent curing agent and a phenolic epoxy resin, and which is polymerized further by UV radiation after having been coated on a substrate material in web form. The pressure sensitive adhesive is thermally postcrosslinkable, and the epoxy resin fractions and curing agent fractions present in the copolyacrylate matrix react with one another. Again, acrylates are also needed in order to produce epoxide adhesive tapes which are pressure-sensitively adhesive and thermally postcurable. While claims are made of up to 60 wt % epoxy resin, the inventive examples only realize <51 wt %.
Known additionally, from J. Appl. Polym. Sci. 41, 467, 1990, “Rubber-Modified Epoxides”, is the addition of elastomers to epoxy resin adhesives. A common method for generating two-phase morphologies, for example in epoxy resin adhesives, is the addition of an endgroup-modified, epoxide-reactive polybutadiene-co-acrylonitrile copolymer to the uncured epoxy resin. In this case the thermoplastic polymer must be soluble in the uncured epoxy resin, but in the course of the curing reaction must be incompatible with the epoxy resin polymer, producing phase separation in the course of curing. When the gel point is reached, the process of phase separation is halted, and so the thermoplastic or elastomeric polymer is present in the form of microscopic spherical domains in the epoxy resin matrix. When elastomers are added to an epoxy resin, therefore, a disperse elastomer phase is formed in the epoxide matrix.
EP 0 927 737 A1 describes the addition of elastomer particles of high melting point (nylon, for example) and carboxy-terminated nitrile rubber in epoxide adhesives. It is shown that even in the cured tape, these particles are present as phase-separated particles with a size of ˜3 μm in the epoxide matrix. According to this specification, the addition of elastomer particles and nitrile rubber is advantageous when they are present as a disperse phase after curing. Furthermore, solid bisphenol A epoxides produce lower crosslinking densities than their liquid derivatives. Such so-called “prepregs” (“preimpregnated fibres”) are also “self-adhesive” and “tacky”, but by definition such prepregs consist of woven fabrics impregnated with epoxide, which give the epoxy resin a certain cohesion.
Also known are sheetlike structures impregnated with liquid resin, such as prepregs, for example, for the production of circuit boards or components made from fibre composite plastics. These are sheetlike textile structures impregnated with a reactive resin. For establishing storability and transportability, the reactive resins are generally partially gelled, meaning that the curing reaction is initiated and then halted at an early stage (B-stage). Here there is a marked increase in viscosity of the resin, allowing the impregnated structure to be handled. Prepregs of this kind are pressure-sensitively adhesive and can therefore be laminated together at room temperature. Like adhesive tapes, they are generally lined with release liners in order to make them stackable or windable. A disadvantage of this prior art is that the partially gelled prepregs must be stored under cool conditions in order to prevent continuation of the curing reaction. Moreover, the increase in viscosity lessens the wetting capacity for the substrate surface on adhesive bonding. Prepregs impact-modified with polyurethane and with polyvinyl formal are described in JP 1991282820A and in U.S. Pat. No. 4,859,533, respectively.
Polyurethanes are used in epoxide mixtures, besides, in order to obtain a very low pressure-sensitive adhesiveness. This is described for example in DE 10 2008 053 520 A1. Even the crosslinked polyurethane composition described in WO 2009 09814 A1, with not more than 50 wt % of epoxide, results in an adhesive which is not pressure-sensitively adhesive.
To summarize: for structural adhesive bonds in the prior art, accordingly, almost exclusively liquid epoxide adhesives or urethane adhesives are used. They are usually blended with impact modifiers consisting of elastomers and/or thermoplastics, to form a disperse elastomer/thermoplastic phase. These systems are not pressure-sensitively adhesive.
Aside from prepregs, in which the woven fabrics provide the epoxy resin with a certain cohesion, the only pressure-sensitively adhesive systems described to date for structural bonds have been those based on acrylates with epoxides. The acrylates usually contain epoxide functionalities and so in the curing step the acrylate is cured via the epoxide crosslinkers. The shear strength rises with the epoxide fraction, but conversely this results in the pressure-sensitive adhesiveness falling off. A condition for the presence of pressure-sensitive adhesiveness is a certain cohesion. This is provided via the pressure-sensitively adhesive acrylates, and, when certain amounts of liquid epoxide are reached, it goes into sharp reverse. A key disadvantage in the present state of the art is the low epoxide content, restricting the shear strengths that can be achieved.